A high electron mobility field-effect transistor (HEMT), or modulation-doped field effect transistor (MODFET), is an extremely fast transistor device that is used in high-speed monolithic microwave integrated circuits (MMICs). Applications for HEMTs include wireless millimeter-wave communications, fiber-radio personal communication systems, automobile collision-avoidance radar, and optical fiber and low-noise direct broadcast satellite (DBS) communications receivers.
A typical HEMT includes a Schottky barrier layer overlying a donor layer, both of which are typically formed of a wide band-gap semiconductor such as an aluminum-containing material. Exposure of the Schottky barrier layer, the semiconductor surface, or the donor layer, to impurities during the fabrication process or during the normal operation of the HEMT can result in the degradation of the semiconductor surface of the Schottky barrier and/or the underlying donor layer. This adversely affects the performance and the reliability of the HEMT.
It should be noted that the term "degradation" is used herein and applies to oxidation, fluorine passivation, and other forms of deterioration due to the presence of impurities.
Various solutions to these degradation problems have been proposed. Most of the solutions typically involve the use of one or more thin epitaxial layers of an aluminum-free semiconductor as "stop layers" to prevent degradation of the Schottky barrier layer and/or the underlying donor layer. For example, U.S. Pat. No. 5,172,197 to Nguyen et al. describes the use of a passivation or stop layer of a lattice-matched, non-oxidizable material forned underlying the source, drain, and gate, and sealingly overlying the donor layer. One drawback of these alternative solutions is that the stop layer tends to increase the gate leakage current, which is undesirable for a good Schottky barrier layer, and adversely affects the electrical performance of the HEMT. A solution, which would prevent degradation of the Schotily barrier layer and/or the donor layer while minimizing any increase in the gate leakage current, has been long sought but has eluded those skilled in the art. As the semiconductor industry is moving to even higher speed applications, it is becoming more pressing that a solution be found.